[log] Improve event logging

[Sxnan]

Linked issue: #444

Purpose of change

• Add a config option to set the event log output path.
• Refine Python event logging format: use JSON eventString from Python, remove wrapper context/sourceTimestamp in logs, and emit a flattened { timestamp, event } structure.

Tests

• org.apache.flink.agents.runtime.eventlog.EventLogRecordJsonSerdeTest
• python/flink_agents/e2e_tests/e2e_tests_integration/python_event_logging_test.py

API

• No public API changes. (Event log output format changed.)

Documentation

• doc-needed
• doc-not-needed
• doc-included

[Sxnan]

@xintongsong Could you review this PR?

[xintongsong]

The changes LGTM. I just wonder, can we also control the order of event fields in the printed logs?

This is what we get now:

{"timestamp":"2026-01-17T03:16:29.500129Z","event":{"id":"304c6961-a8cd-47cc-b31a-cbeed2a8e630","attributes":{},"input":"test input data","eventType":"org.apache.flink.agents.api.InputEvent"}}

It might be nicer to always have the type & id in the beginning.

{"timestamp":"2026-01-17T03:16:29.500129Z","event":{"eventType":"org.apache.flink.agents.api.InputEvent","id":"304c6961-a8cd-47cc-b31a-cbeed2a8e630","attributes":{},"input":"test input data"}}

[xintongsong]

LGTM